This invention relates to a method and apparatus for the non-invasive measurement of the evaporation of the aqueous tear film from the front surface of the eye. The invention also provides a new measure of exposed surface area of an eye.
The eye of humans, as well as other animals, is normally covered by a tear film. An unstable or deficient tear film can cause ocular discomfort and more severe afflictions of the conjunctival and corneal surfaces. The eye loses tear fluid by various mechanisms, including outflow through the puncta and evaporation.
The tear film is subject to evaporation, which can reduce the thickness of the tear film, whenever it is exposed to air of less than one hundred percent relative humidity. This evaporative reduction in thickness prompts the often-used clinical measurement of tear breakup time. However, a consistent measure of tear film evaporation has not been available. Rather, different investigators working with rabbits and using various techniques have reported widely varying evaporation rates, namely from 7.8 to 41.6, the measurements being in the units 10.sup.-7 gm/cm.sup.2 /sec. One prior report for evaporation from the precorneal tear film of a human eye was 27 such units. In contrast, apparatus according to this invention consistently indicates that the evaporation rate from an eye of a healthy human is approximately 4, whereas that of a pathologic human is in the order of 8, using the same units.
One investigator, Von Bahr in 1941, determined the amount of water vapor uptake by circulating dry air in a chamber that was attached by suction to the corneal surface of a rabbit eye. (Konnte der Flussigkeitsabgang durch die Cornea von physiologischer Bedeutung sein?, Acta Ophthalmol. 19, 125-34, von Bahr, G. (1941)) Mishima and Maurice in 1961 calculated the evaporation rate from the precorneal tear film of rabbits from measurements of corneal thickness and made with the anterior eye chamber filled with oil. (The oily layer of the tear film and evaporation from the corneal surface., Exp. Eye Res., 1, 39-45, Mishima, S. and Maurice, D. M. (1961)) Iwata, Lemp, Holly and Dohlman in 1969 studied evaporation from the precorneal tear film and the corneal surfaces of rabbit eyes by sealing a chamber of polymethylmethacrylate on the cornea, and measuring the water vapor taken up by dry air passed through the chamber during a given time period. (Evaporation rate of water from the precorneal tear film and cornea in the rabbit., Invest. Ophthalmol. Vis. Sci., 8, 613-9, Iwata, S., Lemp, M. A., Holly, F. J. and Dohlman, C. H. (1969)). In a 1980 study, Hamano, Hori, and Mitsunaga used a modified dermatologic evaporimeter that operates on the basis of the law of water diffusion into air from a surface. (Application of an evaporimeter to the field of ophthalmology., J. Jpn. Contact Lens Soc., 22, 101-7, Hamano, H., Hori, M. and Mitsunaga, S. (1980)). The Hamano et al experiments involved touching the tear film or the corneal surface. It is considered highly likely, however, that any such contact would interfere significantly with the delicate and unstable structure of the tear film.
It accordingly is an object of this invention to provide a method and apparatus for measuring preocular tear evaporation with precision and accuracy.
A further object of the invention is to provide for the measurement of tear evaporation non-intrusively and non-invasively, i.e. without introducing any instrument or foreign substance to the tear film or the corneal surface.
A more specific object of the invention is to provide a method and apparatus for the foregoing measurement in both normal and pathologic cases under controlled reproducible, physiological conditions and without the disadvantage of any stimulus or perturbation causing irritation, blinking or other alteration of the tear film structure.
It is a further object of the invention to provide an improved measure of the area of an exposed, i.e. open, eye surface. A further aspect of this object is to provide such a measure non-intrusively and non-invasively and, moreover, in a way that is easily practiced.
Other objects of the invention will in part appear hereinafter and will in part be obvious.